Electro-chemical deterioration test method and apparatus

ABSTRACT

An apparatus for deriving data concerning the electro-chemical characteristics of a plurality of individual electro-chemical cells or corrosion processes is disclosed. In one aspect, the apparatus includes means for temporarily applying across a cell or a process a controlled multi-frequency electrical signal; means for detecting returned data resultant from said application, and means for utilizing the returned data with to resolve the data into at least three of its electro-chemical constituents, including a series metallic resistance, an electro-chemical resistance, and a double-layer capacitance.

This is a continuation of U.S. application Ser. No. 10/252,035, entitled“Electro Chemical Deterioration Test Method and Apparatus,” and filedSep. 20, 2002, in the name of Nigel D. Scott, now U.S. Pat. No.6,747,456; which was a continuation of U.S. application Ser. No.09/509,882, entitled “Electro Chemical Deterioration Test Method andApparatus,” and filed Mar. 31, 2000, in the name of Nigel D Scott, nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

THIS INVENTION concerns a method of and apparatus for detecting andevaluating the electro-chemical characteristics of an object under test,and is particularly though not exclusively, concerned with themeasurement of the electrical service capacity of in-circuit sealedelectro-chemical cells and monoblocs.

1. Description of Related Art

The invention is especially concerned with the testing of batterysystems of the type used to provide uninterruptible back-up powersupplies in large installations such as computers, life support machinesand radar controlled safety systems for air transport. Such back-upbatteries are required so that if the mains supply should fail, theymust provide electrical supply to their critical loads instantaneouslyor at least in no more than a millisecond or two.

Such batteries may be composed of several hundred cells with a designlife of up to 12 years and costing perhaps several hundreds of thousandsof pounds.

In-situ testing of such batteries is an essential requirement since suchcells are connected in series or series-parallel, and if only one cellof a standard battery becomes faulty the entire back-up supply may belost.

The failure of such battery systems is quite common causing considerabledamage to the critical loads which they supply.

Managerial personnel of large computer centres, air traffic controlsystems, etc. which depend upon such back-up power supplies generallywill not allow their critical loads to be used to test the system andso, conventionally, all such tests carried out on standby batterysupplies involve the temporary shutting down of dependent systems inorder to avoid consequent damage should the mains supply fail during thetest. Upon shut-down the entire back-up battery is disconnected from theinstallation and connected to a large DC load bank and then submitted toa standard discharge test lasting up to three hours. Such tests arenormally carried cut once or twice per annum and this typically cannecessitate the critical load being off-line for up to three days. Inaddition, conventional testing typically consumes many hundreds ofkilowatts of power, the energy being, “burnt off” as heat which resultsin considerably waste of time and energy resource.

A principal object of the present invention is to provide a portableinstrument capable of testing individual cells or monoblocs in-situ,i.e. without disconnection of the back-up supply and without endangeringthe dependent system should the mains supply fail during the test.

The system should enable discharge testing of a single cell in a batteryof multiple cells, while the cell is still in circuit and with nodisruption to the on-line system. Preferably, after testing an entirebattery and finding it to be, as a whole, in good condition a tertiaryelement of the system will enable sample discharge tests to be conductedon a small number of cells so that the whole battery condition can beextrapolated, obviating the necessity for an annual discharge test andshowing notable savings to the end user in terms of down time, manpowerand energy expenditure. The ability of an instrument to identifydeterioration in a single cell, in batteries of many hundreds of cells,will enable an engineer to target and replace only weak or failing cellsthus greatly extending the working life of the entire battery, savingnot only on costs and manpower but also energy consumption.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method ofderiving data concerning the electro-chemical characteristics of anobject under test, comprising the steps of providing a portableinstrument capable of temporarily applying across the object acontrolled frequency electrical signal derived from an internal batteryof the instrument, detecting returned data resultant from saidapplication and utilising the returned data and a tailored mathematicalalgorithm in dedicated computer software to derive the data required.

Particularly though not exclusively the method is used to measure theservice capacity of individual cells of multi-cell valve regulatedlead-acid batteries while in service, and to store the resultant datafor subsequent analysis.

Still further, the invention concerns an instrument adapted to carry outthe aforesaid method.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram illustrating the internal resistance circuit of abattery cell.

FIG. 2 is a diagram illustrating the conventional system for measuringsimple AC impedance in a cell;

FIG. 3 is a diagram illustrating a system for measuring simple DCresistance in a cell;

FIG. 4 is a diagram illustrating the principal components of aninstrument for performing a testing method in accordance with theinvention;

FIG. 5 is a circuit diagram of a part of the instrument;

FIG. 6 is a circuit diagram of a further part of the instrument.

and FIG. 7 is a circuit diagram of an ancilliary testing module.

DETAILED DESCRIPTION OF THE INVENTION

In order to appreciate the innovative content of an instrument designedin accordance with the present invention it is necessary to consider twoconventional methods used for service capacity measurement of sealedlead acid cells and monoblocs. These consist essentially of measuringthe AC impedance (or AC conductance) and the DC resistance ofelectro-chemical cells. These parameters are known to change with ageand state of charge, rising gradually over the life of the cell, orsharply as the cell reaches the end of a discharge.

The equivalent internal resistance circuit of a cell is illustrated withreference to FIG. 1 where Rm is the metallic resistance, Re is theelectrochemical resistance, C is the total capacitance, and WI is theWarburg impedance, or mass transport impedance, of the cell.

Simple AC impedance is easier to measure accurately in-situ than DCresistance. It is known that cell AC impedance changes with loss ofcharge and capacity, and there are instruments presently available whichuse AC impedance to detect and predict the condition of cells inservice.

FIG. 2 illustrates the principal features of a known instrument todetect AC impedance where an AC signal source is coupled across thebattery as a whole and the measuring instrument is clipped across eachcell or monobloc in turn. The decoupled AC voltage and current throughthe cell is then measured and the simple impedance computed. Since thesignal is AC there is no net current flow. Systems operating in thismanner are considered to be accurate to plus or minus 5% but there areseveral drawbacks with this type of equipment as follows. Simple(non-complex) impedance measurements of cells are subject to manyfactors which may cause to confuse the estimation of capacity. These canbe of such significance that in some circumstances simple impedancebears only an indirect relationship to the ability of a cell to deliverits required current. For example, dendritic growth on the plates of thecell can produce a lowering of the cell impedance, which tends todisguise the raised resistance of an aging cell so that the cell appearsmore healthy than is appropriate.

In addition, the cell capacitance is surprisingly large, more than 1.5Farads per 1 Ah in some cases, and this changes in far greaterproportion than the cell resistance component with the charge/dischargecycles experienced during the lifetime of the cell.

Warburg impedance (mass transport impedance) is present during lowerfrequency measurement.

If these components are not identified and accounted for which is notpossible using simple impedance measurement, then the veracity of themeasurements can be obscured.

Very few battery manufacturers provide impedance values for theirgeneric cell types so that capacity or remaining service life cannoteasily be calculated against a known datum but is only valid whencompared against several cells from the same battery, a cell beingconsidered suspect when it differs from normal by plus or minus 25 to30%.

In sensitive installations it may not be desirable to connect an ACsignal across the DC terminals of a battery since this may influence theinverter or load system and in fact many system engineers will not allowthis type of testing to be carried out without disconnection of theload. Furthermore, mains power is required for an AC testing instrument,making it unsuitable where mains access is difficult.

Considering now the alternative of using DC resistance to test the cellsof a battery this would appear to be the method providing the bestsingle piece of information about the cell since it is held mainly tochange with loss of active surface area on the positive plate and hencethe ability of the cell to deliver its rated capacity. Additionally,many battery manufacturers do provide resistance values for theirgeneric cell types. It is not possible to apply the manufacturers methodof measuring cell internal resistance since the cell under test is incircuit but an alternative method as illustrated in FIG. 3 comprisesattaching the terminals of an instrument across the individual cell ormonobloc terminals, and connecting a low resistance (virtually a shortcircuit) which is internal to the instrument, across the cell forseveral seconds dropping the cell apparent DC terminal voltage andstressing the cell. The current and voltage are measured at the momentbefore disconnection of the resistor, and again the moment after. Thechange in cell volts, over the current, is considered to provide anindication of the cell internal resistance. Claims for the accuracy ofthis method are limited to repeatability which is stated to be plus orminus 5%.

The accuracy of the DC resistance measurement method is dependent upon anumber of factors other than the internal resistance, such as terminalfloat voltage.

DC resistance itself however can give an indication of either the stateof charge of the cell or its general condition but it cannotdifferentiate between these conditions unless readings are taken overlong periods of time when trends can be averaged out. However the systemdoes have the advantage that it can run from an internal battery makingit more flexible than the impedance instrument.

Problems encountered with this method of measuring using simple DCresistance are for example that all capacitors have a leakageresistance, and the electro chemical cell is no exception. Withreference to FIG. 1 it can be seen that if the capacitor C has a DCleakage resistance which is dependent upon the value of the capacitance,then the overall resistance of the cell will be effected and inparticular the electro-chemical resistance, arguably the most importantpart, since if analysed correctly this can indicate such conditions asdrying out, dendritic growth, and plate corrosion due to aging.

Again with the DC resistance method if the cell is stressed during thetest by having 50 to 100 amps drawn from it there is a net current lossand a chemical change in the cell. This has the effect of raising thefloat voltage across the remaining cells of the battery and as the testprogresses there is a noticeable rise in individual cell terminalvoltage, thus invalidating the voltage reading in the later-testedcells.

Furthermore, the method takes no account of capacitance which changesmore radically than DC resistance for a given chemical difference, andcan be an important indicator of both condition and state of charge.

In conclusion, the conventional methods are both somewhatunsatisfactory. AC impedance is the preferred method of acquiring, celldata since it can potentially provide far more information than simpleDC resistance, allowing true analysis of the various factors involved,and since there is no net current flow, the cell under test and theremaining cells in the battery are not effected by resultant chemicalchanges. Such an instrument would be considerably improved if it did notinject its source signal across the entire battery. Simple impedance,however, is not capable of providing sufficient information to makeaccurate judgments about the condition of the cell under test.

Therefore, in accordance with one embodiment of the invention aninstrument is provided which is designed to be portable, i.e. ofsufficiently low mass to be carried while testing, over extended periodsand without strain to the operator. Such an instrument is illustrated inFIG. 4 and is operated by attaching its leads across the terminals of acell under test while the cell is still in circuit, under float chargeconditions. Within the instrument is a component adapted to generate acompound pseudo random noise or cyclic waveform signal which is appliedacross the terminals of the cell. The reaction of the cell is measuredand an original complex mathematical algorithm combined with computersoftware installed within the instrument is used to derive comprehensivedata on the electrical constituents of the cell. The data is stored,together with the DC terminal voltage, float current and cell ambienttemperature readings, and used in a post-test model to indicate thecell's capacity and predict its remaining service life. This informationwill be provided both numerically and graphically.

An active load which can be integrated with the instrument will enablesample cells to be discharge tested to extract parameters which willdetermine absolutely the ability of the cell to support the criticalload when called upon to do so. The data may be subsequently down loadedonto a floppy disc for further evaluation and archiving.

The instrument, in generating pseudo random noise or multi-frequencywaveform employs a technique not previously used in this field and whichwill enable precise measurement of the cell's electro-chemicalconstituents and enable analysis to determine the extent of platecorrosion and electrolyte exhaustion. The test signal when combined withan ultra-fast analogue measurement system and an original algorithmutilising inferential mathematical measurement techniques and others,will, together, derive individual resistive and capacitive componentsfrom the precisely measured feedback.

The operator may not always be able to interpret the raw measured datain such a way as to determine cell status and life criteria. Therefore,in order to provide an authoritative indication of the cell's conditionand probable service life a second mathematical model based on compleximpedance values and others such as DC current and temperature will beprovided so that in addition to the ability to view data and graphicalrepresentations of the cell's characteristics there will be provided asimple graphical interpretation of actual capacity.

The test signal applied to the electro-chemical process may be anyoscillatory current or voltage controlled injected signal, for example,sinusoidal, square wave, sawtooth, step and ramp. The derived signalwill be measured for both current and voltage simultaneously.

When the test signal employed is of an oscillatory nature, an electricalcircuit known as a flying bridge enables the battery internal to theinstrument to be utilised in such a manner as to have zero nett energytransfer (excluding losses) between the cell under test and theinstrument's internal battery.

Referring now to FIG. 5 the operation of the flying bridge is asfollows. When the test signal to be injected into the cell 10 under testis, for example, a sine wave, the positive half cycle is derived fromthe instrument's internal battery 11 via the bridge transistor 13, andapplied to the cell 10, and the negative half-cycle is derived from thecell 10 via the transistor 12 and applied to the instrument's internalbattery 11. Since the cell or monoblock under test can be of a voltagebetween 1 and 12 volts and the internal battery must be of a fixedvoltage, perhaps in the middle of the above range, programmable chargepumps 14 and 15 must be employed to change the voltage levels of the twoalternating sources and enable the internal battery 11 to force currentthrough the cell under test, and vice versa.

The system provides a facility for the battery to be “micro-recharged”from the cell under test, after the test series without extending thetest longer than 8 seconds. In addition the same system will have theability to recharge completely the internal battery 11 over an extendedperiod.

Use is made a new and original inferential measurement technique(mathematical algorithm) to resolve the returned measured compleximpedance data into its equivalent circuit electrical constituents, froma signal applied to the external terminals of the cell. An example of amathematical algorithm for this purpose is given as follows.

It has been shown that the dynamics of a cell can be represented by theRandles equivalent circuit. The transfer function of which is:

$\begin{matrix}{\frac{v(t)}{- {i(t)}} = \frac{{\left( {R_{t} + R_{\Omega}} \right) + {{R\;}_{t}R_{\Omega}C_{dt}s}}\;}{1 + {R_{t}C_{dt}s}}} & (1)\end{matrix}$

The problem to be solved is the explicit identification of theseequivalent circuit parameters. To achieve this, the Laplace transferfunction has to be converted in a form suitable for identification.ν(t)+ν(t)R _(t) C _(dl) s=(R _(t) +R _(Ω))i(t)+R _(t) R _(Ω) C _(dl)si(t)  (2)Rearranging:ν(t)=−ν(t)R _(t) C _(dl) s+(R _(t) +R _(Ω))i(t)+R _(t) R _(Ω) C _(dl)si(t)  (3)

Identification requires measurements of input/output data to be made.This introduces sampling in to the system and the model has to beconverted into a discrete form to incorporate the sampling effect. Thedelta operator has been adopted in this work rather than the shiftoperator, because it has superior numerical properties and thecoefficients in a delta model tend to show a closer resemblance to thecoefficients in the continuous model (allowing easier interpretation ofresults) compared to the use of the a model based on the shift operator.

Thus we define the delta (difference) operator

$\begin{matrix}{{\delta\;{y(t)}} = \frac{{y(t)} - {y\left( {t - 1} \right)}}{\Delta}} & (4)\end{matrix}$where ▴ is defined as the sample time between measurements.

Replacing the Laplace operator (s) with the delta operator δ(t) inequation (3)ν(t)=−δν(t)R _(t) C _(dl)+(R _(t) +R _(Ω))i(t)+R _(t) R _(Ω) C _(dl)δi(t)  (5)Define:K=R _(t) +R _(Ω)τ₁ =R _(t) R _(Ω) C _(dl)andτ₂ =C _(dl) R _(t)resulting in a model which can be identified from input/output data.ν(t)=−δν(t)τ₂ +Ki(t)+τ₁ δi(t)  (6)

Statistical estimation techniques can then be used to estimate theparameter values (τ₁, τ₂, K) of the model from input/output data,generated by applying a current test signal to the cell and logging theresulting voltage output data.

Using standard notation, define:ν(t)=x ^(τ)(t)θ+e(t)  (7)where θ is the vector of unknown parameters defined by:^(τ)=[τ₁Kτ₂]  (8)and e(t) is the regression vector consisting as measured input/outputdata defined by:x ^(τ)(t)=[δν(t)i(t)δi(t)]  9)and e(t) is an unobservable white noise disturbance.

We wish to determine from available data, the vector θ of the truesystem parameters. It is assumed that sufficient operating data has beencollected from the system allowing eqn (7) to be recast in matrix formas:

$\begin{matrix}{\begin{bmatrix}{v(1)} \\\vdots \\{v(N)}\end{bmatrix} = {{\begin{bmatrix}{x^{T}(1)} \\\vdots \\{x^{T}(N)}\end{bmatrix}\hat{\theta}} + \begin{bmatrix}{\hat{e}(1)} \\\vdots \\{\hat{e}(N)}\end{bmatrix}}} & (10)\end{matrix}$where N is the number of observations and the modelling error is acombination of the white noise disturbance and the difference betweenthe actual and estimate parameter values:i.e. ê(t)=e(t)+x ^(τ)(t)(θ−{circumflex over (θ)})  (11)

The aim is to estimate the parameter vector θ using Least Squares.Rewriting eqn (10) in stacked notation:V=X{circumflex over (θ)}+Êwhere

$\begin{matrix}{{V = \left\lbrack {{v(1)}\mspace{14mu}\ldots\mspace{14mu}{v(N)}} \right\rbrack},{{\hat{E}}^{T} = {{\left\lbrack {{\hat{e}(1)}\mspace{14mu}\ldots\mspace{14mu}{\hat{e}(N)}} \right\rbrack\mspace{14mu}{and}\mspace{14mu} X} = \begin{bmatrix}{x^{T}(1)} \\\vdots \\{x^{T}(N)}\end{bmatrix}}}} & (12)\end{matrix}$it can be shown that by minimising the sum of squares of the modellingerrors, the least squares estimator for the unknown parameter vector is:{circumflex over (θ)}=[X^(τ)X]⁻¹[X^(τ)V]  (13)

Relevant implementations issues such as bias on estimates resulting fromnon-white measurement noise and computational efficiency resulting fromrecursive implementations of eqn (13) will be highlighted wherenecessary in the next progress report which will focus on theapplication of the identifier to practical data obtained from an actualbattery.

From equation (13), values for the parameters (τ₁,τ₂, K) are obtained.Using a process of back substitution, values for the equivalent circuitcomponents are obtained from (τ₁,τ₂,K).

i.e.

${R_{\Omega} = \frac{\tau_{1}}{\tau_{2}}},$R₁=K−R_(Ω) and

$C_{dl} = \frac{\tau_{2}}{R_{t}}$

The derived signal to be measured will be measured for both current andvoltage simultaneously. An electrical circuit to enable the system toremove any DC voltage level from the measurement, in order to ensuregreater AC voltage accuracy is illustrated in FIG. 6.

Referring now to FIG. 6, it is required that a few micro-volts ofinjected AC signal can be measured when the DC terminal voltage of thecell or monoblock under test can be up to 15 volts. Therefore, anoriginal circuit to eliminate the DC voltage has been defined. TheIncoming signal to be measured is applied to two similar differentialamplifiers 16 and 17 which pass only the differences between the inputsignals. One of the two resulting AC plus DC signals is passed directlyto a second differential amplifier stage, while the second signal isrouted through a low pass filter 19 to remove the AC signal voltage. Thesecond differential amplifier stage 18 thus detects AC plus DC on oneinput and pure DC at the same potential on the other. Since only thedifference is passed, the final signal output is pure AC and can beamplified for measurement in the usual way. This circuit is importantnot only for battery testing but as an example also for the evaluationof cathodic protection system corrosion, where the installed systemrelies on a continuous low-voltage DC current for its protectivecharacteristics.

A major problem in the testing of on-line cells and monoblocs is whatthe user does when the test instrument indicates that cells are nothealthy. They may be on the borderline, and the engineer will want toknow is they can hold up sufficiently to wait for change-out until thenext maintenance period. Therefore, referring to FIG. 7, there isprovided a small active portable load system, to autonomy (discharge)test a single cell or monobloc while in-circuit, under float chargeconditions.

Several important issues make up the inventive concept upon which thismethod and instrument are based. These may be expressed as follows:

-   1. the application of an electrical current comprised from, for    example, tailored pseudo random noise by a battery-powered    instrument to an electrochemical cell in-situ;-   2. the application of this signal while the cell is in circuit in    series with other cells under float charge conditions;-   3. the use of inferential mathematical measurement techniques and    others to resolve the returned measured data with complex impedance,    into its electrical constituents, from a signal applied to the    external terminals of the cell;-   4. converting the resolved data, in combination with other data such    as DC terminal voltage, cell ambient temperature and cell DC charge    current, into a definitive statement of the performance and    reliability of the cell;-   5. the ability to discharge test an individual cell while in    circuit, under float charge conditions. This will be closely    controlled, and the discharge current, terminal voltage and cell    ambient temperature will be monitored and recorded by the    instrument. The discharge will be of sufficient length, and the    instrument will analyse resultant data and give a definitive    indication as to the capacity of the cell to deliver its required    current for a specified period of time. The load will have the    ability to recharge the cell to within 80% of pre-test levels before    the in-situ float charging system takes over; and-   6. The ability of the instrument to recharge its internal battery    from the cell under test while the cell is in circuit. This will be    done automatically for two or three seconds during the test but can    be later coupled to a supply for an extended period of time for a    complete recharge.

Whilst the method in accordance with the invention, and the instrumentdesigned to carry it out, have evolved from the need accurately to testsealed lead acid battery cells in-situ and in circuit, nevertheless themethod and an instrument appropriately designed to carry it out may alsoapply to many other endeavors such as the detection of deterioration(corrosion) of metallic water pipes and the like, metal in concrete,metal in solution, and metal protective coatings. Conceivably, also, themethod may be applied to the measurement of electro-chemicaldeterioration of any object across which it is possible to apply anelectrical current, the method employing the measurement of complex ACimpedance by injecting an appropriate signal.

1. An instrument, comprising: means for temporarily applying across acell or a process a controlled multi-frequency electrical signal; meansfor detecting returned data resultant from said application, and meansfor utilizing the returned data with to resolve the data into at leastthree of its electro-chemical constituents, including a series metallicresistance, an electro-chemical resistance, and a double-layercapacitance.
 2. An instrument according to claim 1, further comprisingmeans for storing the returned data for subsequent analysis.
 3. Aninstrument according to claim 1, wherein the controlled multi-frequencyelectrical signal is an oscillating signal or pseudo-random noise.
 4. Aninstrument according to claim 1, wherein the returned data is measuredfor both current and voltage simultaneously.
 5. An instrument accordingto claim 4, wherein the DC component of the measured data is removedtherefrom.
 6. An instrument according to claim 1, wherein when thecontrolled multi-frequency electrical signal is sinusoidal, a flyingbridge is included whereby the positive half-cycle is derived from theinstrument's internal battery via the bridge and applied to the objectunder test, and the negative half-cycle is derived from the object undertest, and the negative half-cycle is derived from the object under testand applied to the instrument's internal battery whereby the internalbattery may be recharged from the object under test in a minimal period.7. An instrument according to claim 1, including the use of a secondmathematical model based on complex impedance values and others such asDC current and temperature whereby in addition to the ability to viewdata and graphical representations of the characteristics of the objectunder test there is also provided a simple graphical interpretation ofthe actual characteristics.
 8. An instrument according to claim 1,wherein the instrument is applied to sealed, valve regulated openlead-acid cells to measure the service capacity of individual cells of amulti-cell battery while in service under float charge conditions andcausing an internal battery of the instrument to be recharged from thebattery cells under test.
 9. An instrument according to claim 1, saidinstrument being portable and further comprising an internal battery.10. An instrument according to claim 9, when applied to measure anddefine the electro-chemical deterioration of a battery, the componentsof the instrument being such as to generate substantially no net energytransfer between the battery under test and the internal battery withinthe instrument.
 11. An instrument according to claim 10, having terminalconnectors adapted to be applied to the external terminals of a batterycell to be tested.
 12. An instrument according to claim 10, wherein itscomponents include a flying bridge such that the internal battery of theinstrument transmits a current through the battery under test and thelatter transmits a current through the internal battery.
 13. Aninstrument according to claim 9, including a portable load systemadapted to autonomy (discharge) test a single battery cell or monoblocwhile in circuit, under float charge conditions.
 14. An instrument forderiving data concerning the electro-chemical characteristics of anelectro-chemical cell in circuit or corrosion process in situ,comprising: a multi-frequency electrical signal generator capable oftemporarily applying across the cell or process a controlledmulti-frequency electrical signal; data acquisition means for detectingreturned data resultant from said application, and a dedicated computerwith computer software algorithms capable of resolving the data into atleast three of its electro-chemical constituents, including a metallicresistance, an electro-chemical resistance, and a parallel capacitance.15. An instrument according to claim 14, provided as a portableinstrument of sufficiently low mass to be carried, while testing, overextended periods and without strain to the operator.
 16. An instrumentaccording to claim 14, wherein the controlled multi-frequency electricalsignal is an oscillating signal or is pseudo-random noise.
 17. Aninstrument to claim 14, the components of which, including an internalbattery thereof, are such as to generate substantially no net energytransfer between a battery under test and an internal battery of theinstrument.
 18. An instrument according to claim 14, including aportable load system adapted to carry out an automatic discharge of asingle battery cell or monobloc while in circuit, under float chargeconditions.
 19. An apparatus for deriving data concerning theelectro-chemical characteristics of a plurality of individualelectro-chemical cells or corrosion processes, comprising: a pluralityof sealed, valve regulated open lead-acid cells; an instrument, capableof: temporarily applying across at least one of the cells a controlledfrequency electrical signal to measure the service capacity ofindividual cells while in service under float charge conditions,detecting returned voltage and current data resultant from saidapplication; utilizing the returned data and a tailored mathematicalalgorithm in dedicated computer software to resolve the data, withcomplex impedance, into at least three of its electro-chemicalconstituents, including a series metallic resistance, anelectro-chemical resistance, and a double-layer capacitance; and storingthe data resulting from the application for subsequent analysis.
 20. Anapparatus according to claim 19, wherein the controlled frequencyelectrical signal is an oscillating signal or a pseudo-random noise. 21.An apparatus according to claim 19, wherein when the controlledfrequency electrical signal is sinusoidal, a flying bridge is includedwhereby the positive half-cycle is derived from the instrument'sinternal battery via the bridge and applied to the cell or process, andthe negative half-cycle is derived from the cell or process, and thenegative half-cycle is derived from the cell or process and applied tothe instrument's internal battery whereby the internal battery may berecharged from the cell or process in a minimal period.
 22. An apparatusaccording to claim 19, including the use of a second mathematical modelbased on complex impedance values and others such as DC current andtemperature whereby in addition to the ability to view data andgraphical representations of the characteristics of the cell or processthere is also provided a simple graphical interpretation of the actualcharacteristics.
 23. A method of deriving data concerning theelectro-chemical characteristics of an object under test, comprising thesteps of providing a portable instrument capable of temporarily applyingacross the object a controlled multi-frequency electrical signal derivedfrom an internal battery of the instrument, detecting returned dataresultant from said application and utilising the returned data and anoriginal tailored mathematical algorithm in dedicated computer softwareto resolve the data into at least three of its electro-chemicalconstituents, including a series metallic resistance, anelectro-chemical resistance, and a double-layer capacitance.